1. Field of the Invention
The present invention generally relates to an alternator, mounted in a vehicle such as a motor car or a motor truck, wherein a cooling fan generating a cooling wind is fixed to a rotor so as to be rotated with the rotor.
2. Description of Related Art
A well-known alternator for a vehicle has a cylindrical stator and a rotor disposed within a housing. The rotor is placed within a center hole of the stator so as to face the stator. The alternator electro-magnetically converts a rotational force of the rotor into an alternating current, and the current flows through armature windings of the stator. Cooling fans are fixed to both ends of the rotor in an axial direction of the rotor, respectively. The fans are rotated with the rotor and generate a cooling wind. The wind flows on the surface of field windings of the rotor along the axial direction, cools a rectfier and armature windings of the stator, and is discharged from windows opened in the housing. The alternator sometimes has a fan base between a core of the rotor and each cooling fan, and the fan base adjusts the flow direction of the wind.
Recently, it has been required to reduce the noise in a vehicle compartment, so that an alternator has been required to suppress the generation of sound or noise of a specific frequency zone offensive to the ear. To reduce the noise, Published Japanese Patent First Publication No. H09-154256 discloses an alternator wherein a cooling fan has variable pitch blades arranged at different pitches along a circumferential direction of the rotor. The blades are fixed to seats (i.e., end surfaces in the axial direction) of nail-shaped magnetic poles of a rotor and extend toward the axial direction so as to stand on the rotor. This alternator suppress the generation of sounds of the specific frequency zone by shifting the frequency of generated sounds to frequencies other than the specific frequency zone.
However, in case of the use of the variable pitch blades in a cooling fan, because the nail-shaped magnetic poles are disposed at equal intervals along the circumferential direction, it is required to fix the blades disposed at different pitches to the poles disposed at equal intervals. Therefore, the blades cannot arbitrarily be fixed to the poles, so that it is difficult to weld the fan to the rotor at a sufficient strength. Because the blades standing on the rotor receives a centrifugal force during its high rotation operation so as to be pulled away from the rotor in the radial and axial directions, a welded portion of the fan sometimes comes off from the rotor, or a portion of the fan where stress is concentrated is sometimes cracked.
To avoid these problems, when each blade plate is thickened to be tightly welded to the rotor, the material cost of the fan is undesirably heightened. Further, when ribs additionally added to each blade are welded to the stress concentrated portion, the flow of the wind is disturbed by the ribs so as to increase fan noise. Because positions of the ribs on the seats are restricted, it is difficult to avoid the increase of the noise.
As another technique for tightly fixing a cooling fan to a rotor, welding positions of the fan are adjusted. For example, Published Japanese Utility Model First Publication No. H05-4764 discloses that a welding position of each blade is placed on a line connecting a center position of the blade receiving a centrifugal force during its rotation and a shaft center of a rotor core. Further, a technical publication No. 124-072 published by Denso corporation discloses a fan wherein a welding portion of each blade is placed outside a circular area which has a diameter equal to 68% of an outer diameter of the fan around a shaft axis of a rotor. However, in these techniques, fans having various blade shapes and arrangements cannot be fixed to rotors at sufficient strength, respectively.